<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
    *{
        margin: 0;
        padding: 0;
        outline: 0;
    }
    .parent{
        width: 500px;
    }
    .fl{
        float: left;
        background-color: green;
        width: 100px;
        height: 100px;
    }
    .main{
        height: 100px;
        background-color: pink;
    }
    </style>
</head>
<body>
    <div class="parent">
        <div class="fl"></div>
        <div class="main">213334656321</div>
    </div>
</body>
</html>